Repairs to a sinking cell block at Ngawha Prison could cost taxpayers as much as $2 million.
Stabilising the block, which has sunk 13cm, is thought to involve boring up to 91 giant screws 20 metres into the ground.
After years of denying the prison was sinking, the Corrections Department last year confessed there were structural problems at the $137 million facility, 7km northeast of Kaikohe.
A piling expert has told the Northern Advocate the fix-it bill could hit $2m.
In June, Corrections advertised for bids to carry out the remedial work required.
In response to questions under the Official Information Act, the department said one end of one of the prisoner "pods" at the prison, which opened in March 2005, had "settled" by 13cm.
Assistant general manager of systems and infrastructure Derek Lyons said the facility was experiencing differential settlement in one building, while a nearby building was also being monitored.
"There is no threat to the safety and security of the facility," Mr Lyons said.
"All buildings experience a period of settlement following construction as the soil compresses beneath the weight of the building.
"Differential settlement occurs when one part of the building settles at a different rate to the others.
"The department commissioned an independent report into the causes of the differential settlement. This report found that the soil movement causing the differential settlement is considered unforeseen and uncharacteristic. The independent report confirmed earlier geological reports ... which stated that the site was appropriate for the facility and that the preparation of the site was also carried out appropriately."
The cracking caused by the sinking of the pod had no impact on the building's use but Mr Lyons conceded that some prisoners might have to be moved to other prisons while the work was carried out.
Corrections looked at several options for the work before deciding on a screw pile technique but would not reveal how much it was expected to cost.
However, a piling expert contacted by the Advocate, but who did not want to be named, said the job involved screwing 91 large screw piles at least 20m into the ground.
He estimated the job would cost up to $2 million and he questioned how much the screws would be affected by sulphur in the geothermal ground at Ngawha.
Mr Lyons said the work was likely to take up to eight months. The soil movement below the pod had been independently assessed as "anomalous and unforeseen", so no one was at fault.
A claim had been lodged with the department's insurers to recover the costs, rather than pursuing Mainzeal, the firm that built the jail. He said the screw pile method would be a permanent fix to the issue and was a structural solution commonly used in New Zealand.
Taxpayers face $2m bill to fix sinking prison
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